The present invention relates to a rare earth-iron-based permanent magnet which includes a rare earth composition, boron, and iron as its principal constituents.
In the past, a rare earth-Cobalt (Co)-based magnet has been known for its high performance properties. However, since the maximum energy product (BH).sub.max of the rare earth-Co-based magnet is not large enough, being about 30 MGOe at the most, the strong demand in recent years for more compactness and higher performance in electronic devices makes it desirable to develop a permanent magnet with higher performance. In response to such a demand, development has been ongoing for a permanent magnet that has iron as its principal constituent (as taught in European patent application No. 101552, U.S. Pat. No. 4,402,770, U.S. Pat. No. 4,533,408, U.S. Pat. No. 4,541,877, and others). The iron based permanent magnet includes a rare-earth element (R) such as neodymium (Nd) and boron (B), with the balance essentially of iron (Fe). It makes use of Fe which is less expensive than Co as the principal ingredient, and is capable of producing (BH).sub.max that can exceed 30 MGOe. Therefore, it represents an extremely promising material that can provide a high performance magnet at low cost.
The drawback of the iron-based permanent magent is that the Curie temperature (Tc) is low compared with the rare earth-Co-based permanent magnet, and has inferior temperature versus magnetic characteristics. This will become a serious problem when it is to be used for a DC brushless motor or the like that is operated under harsh conditions such as high temperature and hence an improvement on this aspect has been desired.
As such an improvement, there has been proposed a composition such as R-B-Co-Al-Fe (EPA 106948). The present inventors have also an application 773,547 filed in 1985 now U.S. Pat. No. 4,664,724. However, the addition of Co brings about a deterioration in the magnetic characteristics so that a demand is strong for improving the performance of the rare earth-Fe-based permanent magnet, with development efforts under way at various laboratories.